1. Field of the Invention
The present invention relates to a voltage-frequency converter circuit which has a temperature compensation function and, more particularly, to a voltage-frequency converter circuit using a comparator and for use in heat-wire type air flow quantity measuring apparatuses.
2. Description of Related Art
A heat-wire type air flow measuring apparatus has been used to measure a quantity of intake air of an internal combustion engine. In this apparatuse, an electric heating current which is supplied to a heat-wire to keep the heat-wire at a predetermined temperature varies in dependence on a mass flow quantity of intake air. The heating current is converted into an electric voltage, which in turn is used as an electric output signal indicative of the air flow quantity.
The recent trend to use digital computers for engine control requires the air flow measuring apparatus to produce the electric output signal in digital form, which has less noise signals and may be processed easily by the digital computer. JP-A-2-77618 discloses one type of air flow measuring apparatuses, which includes a voltage-frequency converter circuit for converting an output voltage signal from a heat-wire control circuit into a corresponding frequency for use in the digital control.
In an engine compartment where the heat-wire air flow measuring apparatus is mounted, temperature varies largely depending on running conditions of the engine, causing changes in heat-wire operation characteristics and lowering accuracy in air flow measurement. To cope with this drawback, JP-A 2-77618 also proposes to reduce variations in the output frequency caused by the temperature change, by providing a necessary temperature compensation characteristics on an offset of an amplifier circuit connected to the heat-wire control circuit output and on a gain of the voltage-frequency converter circuit connected to the amplifier circuit.
The inventors, however, concluded from their own study that the output frequency variation caused by the temperature changes depends not on the characteristics change of the heat-wire but rather on a temperature-dependent response time delay of an output signal of a comparator used in the voltage-frequency converter circuit. The response delay is about several micro seconds (.mu.sec.) in the event the temperature changes from normal (20.degree. C.) to high (80.degree. C.). This will result in flow quantity measurement error of about several tens of percent (%). It is therefore necessary to reduce the response time delay of the frequency-voltage converter circuit to less than 1 .mu.sec. so that the air flow measurement error is reduced to a tolerable value, several percent (%), for use in the engine control.